Detective Constable Mackenzie "Mac" Stone suffers blackouts about the night his wife Lucia disappeared three months earlier. He drinks heavily and obsessively searches for her, only knowing that shortly after she left, she emptied their joint bank account and called him one last time from Paris.
John Dean lost his entire memory five years ago in a car crash; since his amnesia is the result of a brain injury, he has no hope of remembering his past. Dean is offered 7,000 Pounds to participate in a Cambridge lab trial which may heal his injuries and restore his memory. Dean turns it down because he does not want to know who he was before the crash. Dean is now happily married to Jenna; he handles the accounts for her newly created yacht design company.
While searching missing person files for his wife, Stone decides that Dean is really a missing IT professional named Paul West. Stone's partner and best friend, Detective Constable Ian Reid, agrees to help investigate. At first it seems unlikely that Dean is West: the men resemble each other, but only slightly. West committed suicide by jumping off a Channel ferry after his wife and stepson were killed in a house fire that was ruled accidental. But West's body was never found, and Stone finds enough clues to convince him that the fire was a premeditated murder and Dean is faking his amnesia.
When Stone hears that there has been an accident at Jenna's company, he rushes to the hospital, accusing Dean of attempted murder. But Dean is the one who was hurt, keeping Jenna from harm. When Dean complains to Reid, Reid reveals that Stone has not been himself since Lucia disappeared. Reid also reveals that Stone, and only Stone, believes that Dean is a man named Paul West, who killed his wife and stepson.
Stone's behavior becomes even more erratic. He seems to see Lucia leave their house. Stone receives an anonymous letter reading "Why did you do it?". Stone gives the letter to Reid to investigate. Stone admits he has waking nightmares, seeing Lucia everywhere. Since he was drunk the night she left, Stone cannot remember what they argued about. He thinks he remembers blood or mud on his clothes afterward. Reid asks Stone if Stone hit Lucia, but Stone cannot remember. Reid discovers that the letter was created on the same kind of computer and printer that Stone owns. Stone receives a second anonymous letter reading "Why did you kill her?". He hides the letter from Reid.
Jenna's new yacht designing business is having financial trouble. She discovers that she is pregnant. Dean agrees to participate in the Cambridge trial for the money. The Cambridge trial discovers that there is no physical damage to Dean's brain; if he ever had amnesia, he does not have it now. Dean does not tell Jenna.
A third anonymous letter is set to Reid at the police station reading "Check Stone joint account". Reid shows it to his boss, Detective Inspector Brennan. Reid expresses his belief that Stone killed Lucia in a jealous rage, when Stone learned Lucia was unfaithful. Brennan asks how Reid can know this, and Reid admits he was having an affair with Lucia. Acknowledging that Reid might not be objective, Brennan tells Reid to investigate Lucia's disappearance. Reid finds no record of Lucia buying a plane or train ticket to Paris; no record of her calling Stone from Paris; and no record she withdrew any money after she disappeared. Reid tells Brennan of Stone's drinking, blackouts and obsession with Dean; Reid expresses his belief that Stone is sending the anonymous letters himself, as a subconscious confession.
Stone's continued investigation puts a strain on Dean's marriage. Jenna suggests terminating the pregnancy and Dean agrees. At the last minute, she can't go through with the abortion.
Stone's investigation of Dean leads him to conclude that Dean has created other fake identities in the past and committed other murders. Stone believes Dean's MO is to find a woman who has just gone through a messy divorce, marry her, convince her to "follow her dreams" by starting a company with him, embezzle as much money as possible from the company, kill her and then fake his own death. Stone believes that Dean kills the women when they discover his secret -- or become pregnant (because a baby's DNA will be evidence tying Dean to his victim).
Brennan performs an illegal search of Stone's house and finds a large bloodstain under the carpet. Brennan has Stone arrested for Lucia's murder. Stone leads the police to the spot where he last remembers seeing Lucia. Instead of finding a body, they discover Lucia herself, alive. She confirms that Stone has not been hallucinating: she has been in the area, trying to get up the nerve to end her marriage and her affair with Reid in a more adult manner. She also confirms to the police that she did empty the joint bank account, book a ticket to Paris under her own name, and called Stone from Paris. She also confirms that Stone hit her on the night she left, which explains the blood stain. Brennan and the other police realize that Dean must have electronically tampered with the evidence of Lucia leaving to frame Stone for her murder.
Dean talks Jenna into taking a vacation, sailing a client's yacht to Saint-Malo, France. In the middle of the English channel, Jenna finds a hidden, waterproof package with a large amount of cash and ID documents for Dean's next identity. Before Dean can kill her, Stone and Brennan arrive in a police boat. As Jenna jumps overboard and is rescued, Stone and Dean fight on the yacht. Dean tauntingly says that Stone will not get what he wants and leaps off the yacht, disappearing under the water. The police cannot find Dean's body.
The film ends with Dean in an unnamed resort, conning another woman into following her dreams. He introduces himself as "Mac Stone".